1. Field of the Invention
The present invention relates to a method of estimating the residual capacity of a battery (secondary cell) such as a lead-acid storage battery, a nickel-cadmium battery, or the like on a real-time basis.
2. Description of the Prior Art
There has heretofore been known a maximum output estimating process as a method of estimating the residual capacity of a battery, as disclosed in Japanese patent publication No. 1-39069, for example.
The maximum output estimating process is based on the fact that the current and voltage characteristics of a battery as it is discharged vary depending on the residual capacity of the battery. The principles of the maximum output estimating process are described below.
FIG. 6 of the accompanying drawings illustrates the principles of the maximum output estimating process for a lead storage battery. In FIG. 6, the relationship between a discharging current and an output voltage of the lead storage battery at any given residual capacity, i.e., the current/voltage characteristic of the battery, is such that the output voltage decreases linearly as the discharging current increases, as indicated by a linear characteristic curve "a" (solid line). When the residual capacity of the battery decreases, the current/voltage characteristic curve moves downwardly from the linear characteristic curve "a" toward the horizontal axis representing the discharging current, as indicated by a linear characteristic curve "b" (dashed line). Such a current/voltage characteristic tendency is generally found on other batteries such as a nickel-cadmium battery as well as the lead-acid storage battery.
If the residual capacity of the battery corresponds to the current/voltage characteristic linear curve "a", and since at any point in time of use the battery is being discharged at an operating point on the current/voltage characteristic linear curve "a", the value of maximum transfer power that can be extracted from the battery may be determined according to the following equation (1): EQU Maximum transfer power value=(1/4).multidot.Vmax.multidot.Imax(1)
where Vmax and Imax are maximum values of the output voltage and the discharging current, respectively, at the operating point on the current/voltage characteristic linear curve "a". The maximum value of the output voltage is determined as an output voltage value at the time the discharging current value in a linear equation which represents the current/voltage characteristic linear curve "a" is "0", and the maximum value of the discharging current is determined as a discharging current value at the time the output voltage value in the linear equation which represents the current/voltage characteristic linear curve "a" is "0".
Because the current/voltage characteristic linear curve "a" tends to vary depending on the residual capacity of the battery, as described above, the maximum transfer power value determined according to the equation (1) and the residual capacity of the battery are correlated to each other such that the maximum transfer power value decreases as the residual capacity of the battery is lowered. Therefore, if the correlation between the maximum output energy value and the residual capacity of the battery is known, then it is possible to estimate the present residual capacity of the battery from the maximum transfer power value thereof.
According to the conventional maximum output estimating process, when the battery is discharged, a plurality of sets, or at least two sets, of discharging current and output voltage values are measured at sufficiently short intervals of time, and a linear equation which represents the current/voltage characteristic linear curve is determined from the measured sets of discharging current and output voltage values. Then, maximum values Vmax, Imax of the output voltage and the discharging current are determined from the linear equation, and the present maximum output or transfer power value is determined from the maximum values Vmax, Imax according to the equation (1). The present residual capacity of the battery is estimated from the determined maximum transfer power value.
When the residual capacity of the battery is estimated according to the conventional maximum output estimating process, since the linear equation representative of the current/voltage characteristic linear curve is determined from the measured sets of discharging current and output voltage values, it is preferable for the measured discharging current values to have relatively large differences. This is because if the measured discharging current values were the same, then it would be impossible to determine a linear equation representative of the current/voltage characteristic linear curve, and if the measured discharging current values had relatively small differences, then determined current/voltage characteristic linear curves would have large variations due to the accuracy with which the discharging current and output voltage values are measured and the accuracy with which the current/voltage characteristic linear curves are calculated. For example, when a plurality of sets of discharging current and output voltage values are measured at the time the discharging current of the battery does not vary substantially, the measured sets P of discharging current and output voltage values concentrate in an area where the differences between the discharging current values are small as shown in FIG. 6. In such a case, although an equation representative of the current/voltage characteristic linear curve is determined using the method of least squares, for example, the current/voltage characteristic linear curve may be determined as indicated by dash-dot lines (c) or (d), resulting in large variations, depending on small changes in the measured sets P. Thus, when the maximum transfer power value of the battery is determined from the current/voltage characteristic linear curve with closely grouped sets P, the residual capacity of the battery cannot properly be estimated.
As disclosed in Japanese patent publication No. 1-39069, the discharging current value of a battery which is mounted on an internal combustion engine-powered automobile varies relatively greatly when the engine is started, and hence it is possible to measure a plurality of sets of discharging current and output voltage values such that the differences between the discharging currents are relatively large. Therefore, it is possible to determine relatively accurately a current/voltage characteristic linear curve which corresponds to the residual capacity of the battery at the time the engine is started. While the engine-powered automobile is running, the battery is charged by the alternator, and hence it is difficult to estimate the residual capacity of the battery according to the maximum output estimating process. However, almost no problem arises as a result of that difficulty because there is no need to recognize the residual capacity of the battery while the battery is being charged.
Estimating the residual capacity of a battery mounted on an electric vehicle according to the above-described conventional maximum output estimating process has suffered the following disadvantages. The residual capacity of a battery mounted on an electric vehicle directly affects the running performance of the electric vehicle such as the distance or range which the electric vehicle can travel. It is therefore extremely important to be able to recognize the residual capacity of the battery from time to time. On the other hand, when the electric vehicle is running at a constant speed, the discharging current from the battery does not vary substantially. Even if an attempt is made to estimate the residual capacity of the battery on the electric vehicle according to the maximum output estimating process, it is often very difficult or impossible to measure a plurality of sets of discharging current and output voltage values such that the differences between the discharging currents are relatively large. Accordingly, as described above, the determined current/voltage characteristic linear curve tends to suffer large variations, and it is difficult to estimate the residual capacity of the battery accurately from time to time.
Such drawbacks experienced with the estimation of the residual capacity of a battery are not limited to electric vehicles, but are found on those vehicles which use batteries under a condition in which the discharging current thereof does not vary substantially.